PROJECT SUMMARY Uterine vascular adaptation with the striking increase of uterine blood flow during gestation is essential for fetal development as well as for cardiovascular well-being of the mother. Although vasodilatation is of critical importance in the adaptation of uterine circulation to pregnancy, the underlying mechanisms are not fully understood. Spontaneous transient outward currents (STOCs) at physiological membrane potentials (? 30-40 mV) of vascular smooth muscle cells fundamentally regulate vascular myogenic tone and blood flow in an organ, as well as arterial pressure. Our preliminary studies in sheep revealed that pregnancy significantly increased STOCs in uterine arterial smooth muscle cells, identifing an exciting regulatory target in uterine vascular adaptation in gestation. STOCs are mainly mediated by large-conductance Ca2+- activated K (BKca) channels. The BKca channel in vascular smooth muscle contains the channel-forming ? + subunit and up to four accessory ?1 subunits that sense Ca2+ sparks via the opening of ryanodine receptors. Our previous studies demonstrated a significant upregulation of BK?1 subunit expression, resulting in increased ?1:? subunit stoichiometry in uterine arteries of pregnant animals. This resulted from a decrease in promoter methylation of the BK?1 subunit gene in uterine arteries. Dynamic regulation of DNA methylation and demethylation is a chief mechanism in epigenetic modifications of gene expression pattern and biological function. Recent studies have suggested a robust mechanism of ten-eleven translocation 1-3 (TET1-3) proteins in active DNA demethylation by converting 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC). Our preliminary studies showed that pregnancy and steroid hormones significantly increased TET1 and TET2 expression and 5hmC abundance at the BK?1 promoter in uterine arteries, suggesting a highly novel mechanism of hormone-mediated active DNA demethylation in uterine vascular adaptation to pregnancy. In addition, the preliminary data suggested that pregnancy resulted in a significant increase in Ca2+ sparks mediated by ryanodine receptors in uterine arteries. With these exciting findings and many highly novel leads, we are now positioned to move the field forward significantly in a manner by launching a new focus of research aiming at the understanding of molecular and epigenetic mechanisms in the regulation of STOCs that are of fundamental importance in uterine arterial vasodilatation during gestation. Thus, the proposed study will test the hypothesis that pregnancy upregulates BK?1 subunit expression via TET-mediated DNA demethylation and increases ryanodine receptor-induced Ca2+ sparks, resulting in heightened STOCs in uterine vascular adaptation to pregnancy. The outcome of proposed study will provide novel insights in molecular mechanisms of uteroplacental adaptation to pregnancy, and will have a major impact in our understanding of pathophysiologic mechanisms underlying pregnancy complications including preeclampsia and fetal growth restriction, caused by maladaptation of uteroplacental circulation. Given that STOCs are fundamentally important in regulating vascular tone and pressure in virtually all vascular beds, revealing epigenetic regulation of STOCs function in vascular smooth muscle will indeed have a much broader impact in the comprehensive understanding of molecular mechanisms in vascular physiology and pathophysiology.